Liquid crystal compositions, polarization selective membranes and liquid crystal displays

ABSTRACT

A novel polarization selective membrane is disclosed. The membrane comprises a liquid crystal molecule having a helical structure and selectively transmits specific polarized light and selectively reflects the other polarized light. The helical axis of the helical structure is practically oriented in the normal direction of the membrane plane, and the angle between the helical axis of the helical structure and the long axis of the liquid crystal molecules is from 5° to 85°. The membrane may be prepared by using a composition comprising a compound represented by Formula (I): wherein X represents a group having an optically active site, M represents a group having at least one aromatic carbocycle or aromatic heterocycle, Y 1  and Y 2  independently represent a bivalent group; R 1  to R 3  independently represent a hydrogen atom or an alkyl group, R represents an alkyl group

FIELD OF THE INVENTION

The present invention relates to liquid crystal compositions forming ahelical structure and suitable for use in various optical films, as wellas polarization selective membranes and polarization selective filmsselectively transmitting specific polarized light and selectivelyreflecting the other polarized light using liquid crystal compositions.

DESCRIPTION OF RELATED ART

Natural light such as sunlight or the light from normal artificial lightsources such as lamps is unpolarized light (randomly polarized light),but polarized components (linearly polarized light, circularly polarizedlight, elliptically polarized light) can be collected therefrom by theuse of a polarizing plate. Collected polarized light can be used invarious optical equipments. Currently widely distributed liquid crystaldisplay devices can be described as devices using the nature ofpolarized light to display images.

FIG. 4 shows a schematic diagram of a typical liquid crystal displaydevice.

The typical liquid crystal display device shown in FIG. 4 comprises asheet-like light emitter as a light source consisting of an edgelighting backlight source 11 at the bottommost face, a reflector 12 forallowing the backlight to successively emerge upward from the bottom anda light guide 13. A liquid crystal cell 16 sandwiched between twoconventional light-absorbing polarizing plates 14 and 15 on both sidesis provided above the light source, whereby the device has animage-displaying function.

Light-absorbing polarizing plates 14 and 15 shown in FIG. 4 aretypically made from a polyvinyl alcohol film. Polyvinyl alcohol-basedpolarizing plates can be prepared by orienting a polyvinyl alcohol filmand adsorbing iodine or a dichroic dye to it. The transmission axis(polarization axis) of the polarizing plates corresponds to thedirection perpendicular to the orienting direction of the film.Light-absorbing polarizing plates transmit only polarized componentsparallel to the polarization axis and absorb polarized components in thedirection orthogonal thereto. Thus, the light utilization efficiency istheoretically 50% or less (practically, further less), and the liquidcrystal display device shown in FIG. 4 also fails to theoreticallyattain a light utilization efficiency of 50% or more with thisarrangement because at least 50% of the light emitted from the lightsource is absorbed by the lower light-absorbing polarizing plate 14.

As described above, the use of light-absorbing polarizing plates inconventional liquid crystal display devices contributes to the decreaseof the light utilization efficiency, and therefore the decrease of thebrightness of the image displayed. Thus, it would be highly desirable todevelop a technique for efficiently converting natural light orunpolarized light into desired linearly polarized light to increase thelight utilization efficiency in various optical devices such as liquidcrystal devices.

In order to improve the light utilization efficiency in polarizingplates, it has been proposed to use light-reflective polarizing platesin place of or in addition to light-absorbing polarizing plates.Light-reflective polarizing plates can be classified into two maintypes. One uses a stack of a plurality of layers having differentrefractive indices as proposed in JP-A 1997-506985 and JP-A 1997-507308(the term “JP-A” as used herein means an “unexamined published Japanesepatent application”). However, this method had problems such asdifficulty in preparing a film stack or sophistication or processing bylaminating to other functional films. The second method uses selectivereflection of cholesteric liquid crystals as disclosed in JP-A1996-271892 and JP-A 1996-271837. This method solved many of theproblems above, but had the disadvantage that it is highly dependent onthe angle of the light incident on the light-reflective polarizingplates and it was difficult to obtain homogeneous reflectioncharacteristics over the entire visible light. It also had thedisadvantage that the temperature must be precisely controlled duringmanufacturing because the helical pitch of many of cholesteric liquidcrystals widely varies with temperature to result in the variation ofthe wavelength range of selective reflection.

JP-A 2000-310780 discloses a liquid crystal film, of which the directionof the helical axis of the smectic liquid crystal phase with a helicalstructure is fixed so as to be nearly vertical to the substrate surface,and an optical element containing the film.

It is well known that a cholesteric phase having a helical structure isinduced by adding an optically active compound to a liquid crystalcomposition showing a nematic phase. Optically active compounds thatinduce a helical structure when they are added to a liquid crystalcomposition consisting of a naturally optically inactive compound arecalled chiral agents. The pitch length of the helical structuredecreases as the concentration of the chiral agent increases, and thepitch length of a composition containing a different type of chiralagent at the same concentration varies with the structure of the chiralagent. When a chiral agent is added at the same concentration, thehelical twisting power (htp) of the chiral agent tends to be stronger asthe pitch length of the induced helical structure becomes shorter. Thehelical twisting power (β) is generally expressed as β=1/(c×P) where crepresents the fraction of the chiral agent and P represents the pitchlength. Thus, chiral agents should preferably have a high helicaltwisting power in order to control physical properties of liquid crystalcompositions because such compounds can induce a helical structurehaving a short pitch length with small amounts.

It is also known that liquid crystal compositions having a helicalstructure are formed by using an optically active compound or mixing anoptically inactive compound with an optically active compound in a phaseof a liquid crystal compound longitudinally tilted from the normal lineof the smectic phase, such as smectic C phase. For example, JP-A1995-118202, JP-A 1996-120271 and JP-A 1996-291148 disclose that achiral smectic C phase having a helical structure can be induced byadding a chiral agent to a liquid crystal composition showing a smecticC phase. However, any composition capable of reflecting visible light tosuit the object of the present invention is not disclosed. JP-A1997-506088, JP-A 1998-158268, JP-A 1999-193287 and JP-A 2000-515496 andZ. Naturforsch, Vol. 44a, pp. 675-679 (1989) disclose carbohydratederivatives as chiral agents for inducing a cholesteric phase, but referto nothing about other phases than cholesteric phases. Z. Naturforsch,Vol. 43a, pp. 1119-1125 (1988) describes a chiral smectic C phasecontaining a carbohydrate derivative, but refers to nothing about theuse of its selective reflection. On the other hand, chiral agents havinga low helical twisting power must be added in large quantities so thatthe freedom of controlling physical properties of liquid crystalcompositions is disadvantageously limited. Even compounds having a highhelical twisting power in the cholesteric phase were sometimesincompatible with smectic liquid crystals or destabilized the smecticphase when they were added to smectic liquid crystals. Thus, there aredemands for chiral agents having a high helical twisting power in liquidcrystal phases other than the cholesteric phase and liquid crystalcompositions containing small amounts of such chiral agents to have ahelical structure whose pitch length is sufficiently short toselectively reflect visible light.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a liquid crystalcomposition capable of stably forming a helical structure and showingdesired optical characteristics. Another object of the present inventionis to provide a novel polarization selective membrane and polarizationselective film contributing to the improvement of the light utilizationefficiency of polarizing plates, and particularly to provide a novelpolarization selective membrane and polarization selective film showinghomogeneous and high reflection characteristics over a large area inwhich the reflection characteristics of the polarization selectivemembrane are easy to control and the film is readily sophisticated bylaminating to other functional films to show excellent manufacturingsuitability.

In one aspect, the present invention provides a polarization selectivemembrane comprising a liquid crystal molecule having a helicalstructure, selectively transmitting specific polarized light andselectively reflecting the other polarized light, wherein the helicalaxis of the helical structure is practically oriented in the normaldirection of the membrane plane and the angle formed by the orientationof the helical axis of the helical structure and the orientation of thelong axis of the liquid crystal molecules is from 5° to 85°.

As embodiments of the present invention, there are provided thepolarization selective membrane according to claim 1, having a maximumtotal transmittance of 75% or more and a minimum total transmittance ofless than 60% at a polarizing plane perpendicular to the membrane plane;and the polarization selective membrane wherein liquid crystal moleculeshave a chiral smectic phase.

In another aspect, the present invention provides a polarizationselective film comprising:

a transparent support and

a layer comprising liquid crystal molecules having a helical structureand selectively transmitting specific polarized light and selectivelyreflecting the other polarized light, wherein the helical axis of thehelical structure is practically oriented in the normal direction of themembrane plane and the angle formed by the orientation of the helicalaxis of the helical structure and the orientation of the long axis ofthe liquid crystal molecules is 5° to 85°.

In another aspect, the present invention provides a method forpolarizing light comprising entering light into the polarizationselective film to transmit specific circularly polarized light in anincident light and to reflect the other circularly polarized light,reflecting the reflected light by a reflector also serving as apolarization converting layer to enter it again into the polarizationselective film, and converting the circularly polarized light emergingfrom the polarization selective film into linearly polarized light via aretardation layer.

In another aspect, the present invention provides an apparatus forpolarizing light comprising a sheet-like light emitter, a polarizationselective film and a retarder arranged in this order wherein thesheet-like light emitter comprises a light guide having a light sourceon the side face and a reflective layer at the bottom, the polarizationselective film is the polarization selective film and the retarder has aphase change of 100 to 200 nm.

In another aspect, the present invention provides a polarizing platecomprising the polarization selective film, a retarder having a phasechange of 100-200 nm and a light-absorbing polarizing plate laminated inthis order wherein the direction having a polarization plane on whichthe total transmittance of the polarization selective film is maximumand the direction of the transmission axis of the light-absorbingpolarizing plate is substantially parallel.

In another aspect, the present invention provides a liquid crystaldisplay device comprising a backlight, a polarization selective film, aretarder and a liquid crystal cell sandwiched between a pair oflight-absorbing polarizing plates arranged in this order wherein thebacklight comprises a light guide having a light source on the side faceand a reflective layer at the bottom, the polarization selective film isthe polarization selective film and the retarder has a phase change of100 to 200 nm.

As embodiments of the present invention, there are provided the devicewherein the polarization selective film, the retarder and thelight-absorbing polarizing plate near to the backlight are integrated;the device comprising a light scattering sheet and a light-collectingfilm between the backlight and the polarization selective film; thedevice wherein the polarization selective film comprises ananti-reflection layer on the surface near to the backlight; and thedevice wherein the a polarizing plate direction, in which thepolarization selective film has a maximum total transmittance, issubstantially parallel to the transmission axis of the light-absorbingpolarizing plate near to the backlight.

In another aspect, the present invention provides a liquid crystalcomposition comprising at least one liquid crystal and an opticallyactive compound, capable of forming a helical structure in which theangle formed by the orientation of the helical axis and the orientationof the long axis of the liquid crystal molecules is from 5° to 85° andthe optically active compound is a compound of formula (I) below:

wherein X represents a group having an optically active site, Mrepresents a group having at least one aromatic carbocycle or aromaticheterocycle, Y¹ and Y² independently represent —O—, —S—, —C(═O)O—,—OC(═O)—, —OC(═O)O—, —C(═O)N(R¹)—, —N(R¹)C(═O)—, —(CR²R³)_(m)O—,—SO₂N(R¹)—, —N(R¹)SO₂— or —S(═O)_(p)—; R¹, R² and R³ independentlyrepresent a hydrogen atom or an optionally substituted alkyl group; mrepresents an integer of any of 1 to 12, p represents an integer of anyof 0 to 2, m¹ and m² independently represents 0 or 1, provided that whenm¹ and m² are each 0, M and X and M and R are directly bonded; Rrepresents an optionally substituted alkyl group and n represents aninteger of any of 2 to 6.

As embodiments of the present invention, there are provided thecomposition further comprising a liquid crystal compound; thecomposition wherein X in Formula (I) is a cyclic group having anoptically active site; the composition wherein X in Formula (I) is acyclic group selected from Group I:

the composition wherein M in Formula (I) is a group represented byFormula (II) bellow:

Wherein T¹ and T² independently represent an aromatic carbocycle,aromatic heterocycle or aliphatic carbocycle, and at least onerepresents an aromatic carbocycle or an aromatic heterocycle; Y³independently represents —O—, —C(═O) O—, —OC(═O)—, —CH₂O—, —OCH₂—,—CH═N— or —N═CH—; m³ represents 0 or 1, provided that when m³ is 0, T¹and T² are directly bonded; and r represents an integer of any of 0 to3; the composition wherein the helical structure is based on a chiralsmectic phase.

In another aspect, the present invention provides a polarizationselective membrane comprising the liquid crystal composition having ahelical structure with the helical axis practically oriented in thenormal direction of the membrane plane and selectively transmittingspecific polarized light and selectively reflecting the other polarizedlight.

In another aspect, the present invention provides a polarizationselective film comprising:

a transparent support and

a layer comprising the liquid crystal composition having a helicalstructure with the helical axis practically oriented in the normaldirection of the membrane plane and selectively transmitting specificpolarized light and selectively reflecting the other polarized light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an embodiment of a liquid crystaldisplay device using a polarization selective film of the presentinvention.

FIG. 2 is a schematic diagram showing another embodiment of a liquidcrystal display device using a polarization selective film of thepresent invention.

FIG. 3 is a schematic diagram showing a still another embodiment of aliquid crystal display device using a polarization selective film of thepresent invention.

FIG. 4 is a schematic diagram showing the structure of a conventionalliquid crystal display device.

DETAILED DESCRIPTION OF THE INVENTION

[Polarization Selective Membranes and Polarization Selective Films]

The present invention provides a polarization selective membranecomprising liquid crystal molecules having a helical structure andselectively transmitting specific polarized light and selectivelyreflecting the other polarized light, characterized in that the helicalaxis of the helical structure is practically oriented in the normaldirection of the membrane plane and the angle formed by the orientationof the helical axis of the helical structure and the orientation of thelong axis of the liquid crystal molecules is 5° to 85°.

The present invention also provides a polarization selective filmcharacterized in that the polarization selective membrane of the presentinvention is formed as a lamellar polarization selective layer on atransparent support.

In the present invention, liquid crystal molecules are oriented at anangle of 5° to 85° (preferably 15° to 80°, more preferably 20° to 80° )between the orientation of the helical axis and the orientation of thelong axis of the crystal liquid molecules to form a helical structure.The helical structure may be formed by a single liquid crystal moleculeor a mixture of a plurality of liquid crystal molecules or a mixture ofa liquid crystal and a non-liquid crystalline compound. The liquidcrystal phase shown by these liquid crystal molecules or mixture ispreferably a smectic phase, especially a chiral smectic phase,particularly chiral smectic C phase, chiral smectic F phase or chiralsmectic I phase, among which chiral smectic C phase is especiallypreferred. However, liquid crystal molecules may not be optically activeto form a helical structure, but liquid crystals forming a helicalstructure without having any optical active site may be used as shown inJ. Mat. Chem., Vol. 7, p. 1307 (1997), for example.

The smectic liquid crystal phase having a helical structure has astructure consisting of a stack of smectic layers like normal smecticliquid crystal phases, but the orientation of the long axis of theliquid crystal molecules is tilted at an angle from the directionperpendicular to each smectic layer and the tilt direction is shiftedlittle by little from one layer to the next layer to form a helicalstructure. A preferred embodiment of the polarization selective membraneof the present invention comprises a liquid crystal layer maintaining asmectic phase having a helical structure, i.e. liquid crystal moleculesare oriented with the long axis being tilted at 5 to 85° with respect tothe helical axis and the tilt direction is shifted little by little fromone smectic layer to the next smectic layer to form a helical structure.

Liquid crystal molecules used in the present invention are preferablyobtained by adding a chiral agent or introducing an optically activeunit into a liquid crystalline compound as described above in order toshow a smectic liquid crystal phase having a desired helical structure.For example, liquid crystal compounds capable of showing a chiralsmectic phase more likely to form a helical structure such as chiralsmectic C phase, chiral smectic I phase or chiral smectic F phase can beobtained by adding a chiral agent to a liquid crystal compound showingsmectic C phase, smectic I phase, smectic F phase or the like orintroducing an optically active unit into said liquid crystal compound.The helical pitch can be controlled by appropriately adjusting theamount of the chiral agent to be added, the amount of the opticallyactive unit to be introduced, the optical purity, the temperaturecondition during orientation or other factors, and even characteristicsof the film such as selective reflection wavelength in the case for useas a selective reflection element can be controlled.

Whether the helical structure is right-handed or left-handed depends onthe chirality of the chiral agent or optically active unit used, andeither right or left-handed helical structure can be prepared dependingon the chirality selected.

Liquid crystals showing a (chiral) smectic phase and chiral agents arepreferably compounds of formula (I) below. Optically active compounds offormula (I) below can form the helical structure described above aloneor in combination with optically inactive liquid crystalline compoundsand/or optically active liquid crystalline compounds. That is, compoundsof formula (I) below contribute to the formation of the helicalstructure described above mainly as chiral agents and/or liquidcrystalline compounds.

Especially, the polarization selective membrane is preferably preparedfrom a liquid crystal composition containing an optically inactiveliquid crystalline compound and an optically active compound of formula(I) below. In this embodiment, the helical pitch can be easilycontrolled by appropriately adjusting the amount of the optically activecompound to be added, the amount of the optically active unit to beintroduced, the optical purity, the temperature condition duringorientation or other factors. When the membrane is used to form anoptical film or the like, therefore, characteristics of the film such asselective reflection wavelength in the case for use as a selectivereflection element can be easily controlled.

Next, optically active compounds of formula (I) below are explained indetail.

wherein X represents a group having an optically active site, Mrepresents a group having at least one aromatic carbocycle or aromaticheterocycle, Y¹ and Y² independently represent —O—, —S—, —C(═O)O—,—OC(═O)—, —OC(═O)O—, —C(═O)N(R¹)—, —N(R¹)C(═O)—, —(CR²R³)_(m)O—,—SO₂N(R¹)—, —N(R¹)SO₂— or —S(═O)_(p)— where R¹, R² and R³ independentlyrepresent a hydrogen atom or an optionally substituted alkyl group, mrepresents an integer of any of 1 to 12, and p represents an integer ofany of 0 to 2. m¹ and m² independently represent 0 or 1, provided thatwhen m¹ and m² are each 0, M and X and M and R are directly bonded.

In the formula, R represents an optionally substituted alkyl group. nrepresents an integer of any of 2 to 6, preferably an integer of any of2 to 4, more preferably 2 or 3.

In formula (I), X represents a group having an optically active site. Xmay contain one or more optically active sites. Also taking into accountavailability, preferred examples are sugars, optically active di- orpolyhydric alcohols, di- or polycarboxylic acids, hydroxycarboxylicacids, amino acids, binaphthyl derivatives and biphenyl derivatives, aswell as the structures mentioned as preferred examples for X in JP-A1997-506088. Other preferred examples include the following structures.

X may be further substituted by the groups exemplified above. Theexamples of the substituents include hydroxy, halogen atom (such as Cl,Br, F and I), cyano, nitro, carboxyl, sulfo, chain or cyclic alkylgroups having C₁₋₂₀ (such as methyl, ethyl, isopropyl, n-butyl, n-hexyl,cyclopropyl cyclohexyl, 2-hydroxyethyl, 4-carboxybutyl, 2-methoxyethyl,2-diethylaminoethyl), alkenyl groups having C₁₋₂₀ (such as vinyl, allyl,2-hexenyl), alkynyl groups having C₂₋₂₀ (such as ethynyl, 1-butynyl,3-hexynyl), aralkyl groups having C₇₋₁₂ (such as benzyl, phenethyl),aryl groups having C₆₋₁₀ (such as phenyl, naphthyl, 4-carboxyphenyl,4-acetoamidephenyl, 3-methanesulfoneamidephenyl, 4-methoxyphenyl,3-carboxyphenyl, 3,5-dicarboxyphenyl, 4-methanesulphoneamidephenyl,4-butanesulfoneamidephenyl), acyl groups having C₁₋₁₀ (such as acetyl,benzoyl, propanoyl, butanoyl), alkoxycarbonyl groups having C₂₋₁₀ (suchas methoxycarbonyl, ethoxycarbonyl), aryloxycarbonyl groups having C₇₋₁₂(such as phenoxycarbonyl, naphtoxycarbonyl), carbamoyl groups havingC₁₋₁₀ (such as non-substituted carbamoyl, methylcarbamoyl,diethylcarbamoyl, phenylcarbamoyl), alkoxy groups having C₁₋₂₀ (such asmethoxy, ethoxy, butoxy, methoxy ethoxy), aryloxy carbonyl groups havingC₆₋₁₂ (such as phenoxy, 4-carboxyphenoxy, 3-methylphenoxy, naphtoxy),acyloxy groups having C₂₋₁₂ (such as acetoxy, benzoyloxy), sulfonyloxygroups having C₁₋₁₂ (such as methyl sulfonyloxy, phenyl sulfonyloxy),amino groups having C₀₋₁₀ (such as non-substituted amino, dimethylamino,diethylamino, 2-carboxyethylamino), acylamino groups having C₁₋₁₀ (suchas non-substituted acetamide, benzamide), sulfonylamino groups havingC₁₋₂₀ (such as methylsulfonylamino, phenylsulfonylamino,butylsulfonylamino, n-octylsulfonylamino), ureido groups having C₁₋₁₀(such as non-substituted ureido, methylureido), urethane groups havingC₂₋₁₀ (such as methoxycarbonylamino, ethoxycarbonylamino), alkylthiogroups having C₁₋₁₂ (such as methylthio, ethylthio, octylthio), arylthiogroups having C₆₋₁₂ (such as phenylthio, naphthylthio), alkylsulfonylgroups having C₁₋₂₀ (such as methylsulfonyl, butylsulfonyl),arylsulfonyl groups having C₇₋₁₂ (such as phenylsulfonyl,2-naphtylsulfonyl), sulfamoyl groups having C₀₋₂₀ (such asnon-substituted sulfamoyl, methylsulfamoyl), heterocyclic groups (suchas 4-pylidyl, piperidino, 2-furyl, furfuryl, 2-thienyl, 2-pyrrolyl,2-quinolylmorpholine).

More preferably, X represents a group having any of the followingstructures.

In the formula, B¹ and B² independently represent a C1-4 alkyl groupwhich may be substituted or contain —O— (i.e. which may be substitutedby an alkoxy group), an optionally substituted phenyl group or anoptionally substituted carboxyl group.

More preferably, X represents a cyclic group having an optically activesite shown below.

In the formula (I) above, M represents a group having at least onearomatic carbocycle or aromatic heterocycle.

The aromatic carbocycle is preferably a benzene, naphthalene, anthraceneor phenanthrene ring, more preferably a benzene or naphthalene ring,most preferably a benzene ring. When M represents a group having abenzene ring or a naphthalene ring, Y¹ and Y² are preferably attached toM at the position where Y¹ and Y² are in a line, e.g., Y¹ and Y² arepreferably attached to a benzene ring at the 1,4-position or anaphthalene ring at the 2,6-position.

The aromatic heterocycle is preferably a 6-membered ring containing 1 to3 nitrogen atoms (e.g., a pyridine, pyridazine, pyrimidine, pyrazine ortriazine ring) or a 5-membered ring containing an N atom, O atom or Satom (e.g., a pyrrole, imidazole, furan, oxazole, 1,3,4-oxadiazole,thiophene, thiazole or 1,3,4-thiadiazole ring), more preferably apyridine, pyridazine, pyrimidine, pyrazine, 1,3,4-oxadiazole, thiopheneor 1,3,4-thiadiazole ring, still more preferably a pyridine, pyridazine,pyrimidine or pyrazine ring. When M represents a group having anaromatic heterocycle, Y¹ and Y² are also preferably attached to M at theposition where Y¹ and Y² are in a line, e.g., Y¹ and Y² are preferablyattached to a pyridine, pyrimidine, pyrazine, 1,3,4-oxadiazole,thiophene or 1,3,4-thiadiazole ring at the 2,5-position.

These aromatic carbocycles or aromatic heterocycles may have asubstituent including those mentioned as substituents for X.

M preferably represents a group of formula (II) below.

In the formula (II), T¹ and T² independently represent an aromaticcarbocycle, aromatic heterocycle or aliphatic carbocycle, and at leastone represents an aromatic carbocycle or an aromatic heterocycle.

The aromatic carbocycle and aromatic heterocycle represented by T¹ andT² are as defined for the aromatic carbocycle and aromatic heterocyclecontained in M and extend a similar preferred range.

The aliphatic carbocycle is preferably a cyclohexane ordecahydronaphthalene ring. Y³ independently represents —O—, —C(═O)O—,—OC(═O)—, —CH₂O—, —OCH₂—, —CH═N— or —N═CH—, and m³ represents 0 or 1.When m³ is 0, T¹ and T² are directly bonded.

In the formula (II) above, r represents an integer of any of 0 to 3,preferably 0 to 2, more preferably 0 or 1.

In the formula (I) above, Y¹ and Y² independently represent —O—, —S—,—C(═O)O—, —OC(═O)—, —OC(═O)O—, —C(═O)N(R¹)—, —N(R¹)C(═O)—,—(CR²R³)_(m)O—, —SO₂N(R¹)—, —N(R¹)SO₂— or —S(═O)_(p)—. m¹ and m²independently represent 0 or 1, provided that when m¹ and m² are each 0,M and X and M and R are directly bonded.

R¹, R² and R³ independently represent a hydrogen atom or an optionallysubstituted alkyl group. R¹ preferably represents a hydrogen atom or aC1-6 alkyl group, more preferably a hydrogen atom or a C1-4 alkyl group.R² and R³ preferably represent a hydrogen atom or a C1-4 alkyl group,more preferably a hydrogen atom.

m represents an integer of any of 1 to 12, preferably any of 1 to 6,more preferably any of 1 to 4, most preferably 1 or 2.

p represents an integer of any of 0 to 2, preferably 0 or 2.

In the formula (I) above, R represents an optionally substituted alkylgroup. The alkyl chain may be branched or may contain an oxygen atom(—O—), sulfur atom (—S—) or the like (i.e. may be substituted by analkoxy or alkylsulfanyl group). These substituents include thosementioned as substituents for X. The number of carbon atoms contained inR is preferably 1 to 30, more preferably 6 to 20, still more preferably6 to 15. R may also have a polymerizable group such as an acryloyloxygroup or a methacryloyloxy group.

Specific examples of compounds of formula (I) above are shown below, butoptically active compounds used in the present invention are not limitedto the specific examples below.

R Y² M Y¹ n X  1 (n)C₁₂H₂₅ —O—

—COO— 2

 2

—COO—

—COO— 2

 3 (n)C₁₂H₂₅ —O—

—COO— 2

 4 (n)C₁₂H₂₆ —OCO—

—COO— 2

 5 (n)C₈H₁₇ —O—

—CH₂O— 2

 6

—O—

—COO— 2

 7 (n)C₁₂H₂₅ —O—

—COO— 2

 8 (n)C₁₄H₂₉ —S—

—CH₂O— 2

 9 (n)C₈H₁₇OCH₂CH₂ —O—

—COO— 2

10

—COO—

—CH₂O— 2

11 (n)C₁₂H₂₅ —O—

—OCO— 2

12 (n)C₅H₁₁CO₂CH₂CH₂ —O—

—OCO— 2

13 (n)C₁₂H₂₅ —OCO—

—COO— 2

14

—O—

—CH₂CH₂O— 2

15 (n)C₈H₁₇OCOCH₂ —OCO—

—COO— 2

16 (n)C₁₂H₂₅ —O—

—COO— 2

Compounds of formula (I) above can be synthesized by the processdescribed in e.g. German Patent (DE-A) No. 3917196, JP-A 1997-506088 andJP-A 2000-515496.

In the present invention, optically active compounds of formula (I)above may be used alone or in combination with optically inactivecompounds or other optically active compounds, as described above. Twoor more compounds of formula (I) above may be used in combination.Materials used in combination with optically active compounds of formula(I) above are preferably liquid crystalline compounds showing a (chiral)smectic phase and chiral agents. These materials include those describedin “Ferroelectric liquid crystal displays and materials” (published byCMC, edited by Fukuda, 1992) and “Handbook of liquid crystals” (Maruzen,edited by Editorial committee for Handbook of liquid crystals, 2000, pp.267-330). The relationship between the pitch length of the helicalstructure or the birefringence of liquid crystal molecules and thecenter wavelength and the half bandwidth of selective reflection spectrais described in “Photonics series 9, Structure and physical propertiesof ferroelectric liquid crystals” (Fukuda and Takezoe, CoronaPublishing, 1990), p. 285, according to which the half bandwidth ofreflection spectra increases with the birefringence of the liquidcrystal composition. For the object of the present invention, it isdesirable to selectively reflect a wavelength band as broad as possible,and therefore, liquid crystal compounds used in the present inventionpreferably have a birefringence of 1.5 or more, more preferably 1.8 ormore, still more preferably 2.0 or more.

Examples of liquid crystalline compounds and chiral agents that can beused in the present invention are shown below, but materials used in thepresent invention are not limited to the specific examples below.

Optically active compounds of formula (I) above can induce a helicalstructure having a short pitch length to show desired polarizationselectivity with small amounts because of the high helical twistingpower of the chiral agents. When optically active compounds of formula(I) above are combined with optically inactive liquid crystallinecompounds, a helical structure showing polarization selectivity can beinduced by adding about 1 to 20% by mass of the optically activecompounds to the optically inactive liquid crystalline compounds.

However, it is not excluded that optically active compounds of formula(I) above are added beyond the range shown above, and optically activecompounds of formula (I) above are preferably contained at about 1 to30% by mass when they are combined with liquid crystalline compounds inthe present invention.

The polarization selective membrane of the present invention can beprepared by various processes, e.g. by developing the liquid crystalcomposition on a substrate such as glass or a plastic film and dryingand then stripping it. The polarization selective film of the presentinvention can be prepared by developing the liquid crystal compositionon a transparent support such as a plastic substrate and drying it.

Examples of the plastic substrate include, but are not specificallylimited to, plastic film substrates made of polyimide, polyamide-imide,polyamide, polyether imide, polyether ether ketone, polyether ketone,polyketone sulfide, polyether sulfone, polysulfone, polyphenylenesulfide, polyphenylene oxide, polyethylene terephthalate, polybutyleneterephthalate, polyethylene naphthalate, polyacetal, polycarbonate,polyallylate, acrylic resins, methacrylic resins, polyvinyl alcohol,polyethylene, polypropylene, poly-4-methylpentene-1 resins, celluloseplastics such as triacetyl cellulose, epoxy resins, phenol resins,polymer liquid crystals; these substrates having another coating such asa polyimide, polyamide, polyvinyl alcohol or silicone film; and theseplastic substrates surface-treated with a silane coupling agent, metalcomplex such as chromium, lecithin or CTAB(cetyltrimethylammoniumbromide). When the manufacturing process involves sandwiching the liquidcrystal composition between a plurality of plastic substrates,homogeneous or heterogeneous plastic substrates may be combined. Amongthese plastic substrates, film-like substrates are preferably used.

These plastic substrates may or may not have been subjected to anorientation treatment such as rubbing.

The liquid crystal composition may be developed on the plastic substrateby directly applying the liquid crystal composition without using asolvent or applying a solution of the liquid crystal compositiondissolved in a suitable solvent and then evaporating the solvent.

The solvent can be appropriately selected depending on the nature,composition and other factors of the material such as the liquidcrystalline compounds and optically active compounds described above.

The examples of the solvents generally include halogenated hydrocarbonssuch as chloroform, dichloromethane, carbon tetrachloride,dichloroethane, tetrachloroethane, trichloroethylene,tetrachloroethylene, chlorobenzene and o-dichlorobenzene; phenols suchas phenol and p-chloro phenol; aromatic hydrocarbons such as benzene,toluene, xylene, methoxybenzene and 1,2-dimetoxybenzene; alcohols suchas isopropanol and tert-butanol; glycols such as glycerin,ethyleneglycol and trimethylene glycol; glycol ethers such as ethyleneglycol monomethyl ether, diethylene glycol dimethyl ether, ethylcellosolve and butyl cellosolve; acetone, methyl ethyl ketone, methylisobutyl ketone, cyclohexane, ethyl acetate, 2-pyrrolidone,N-methyl-2-pyrrolidone, pyridine, triethylamine, tetrahydrofuran,dimethylformamide, dimethylacetamide, dimethyl sulfoxide, acetonitrile,butylonitrile, carbon disulfide and mixtures thereof.

If desired, a surfactant may be added to the solvent to control thesurface tension of the solution or improve coatability or for otherpurposes.

The concentration of the material in the solution can be appropriatelycontrolled depending on the nature and solubility of the material, thethickness of the film to be prepared and other factors. Theconcentration of the entire solute (i.e. the composition containing aliquid crystalline compound and optionally additives such as a chiralagent, hereinafter sometimes collectively referred to as “liquid crystalcomposition”) in the solution is preferably 0.5 to 70% by mass, morepreferably 1 to 50% by mass. Normally, the concentration of the liquidcrystalline compound in the solution is preferably 3 to 50% by mass,more preferably 5 to 30% by mass.

The coating means include, but are not specifically limited to, spincoating, roll coating, printing, dip coating, curtain coating, Meyer barcoating, doctor blade coating, knife coating, die coating, gravurecoating, microgravure coating, offset gravure coating, lip coating,spray coating and extrusion coating (U.S. Pat. No. 2,681,294). Two ormore layers may be simultaneously applied. Simultaneous coating isdescribed in U.S. Pat. Nos. 2,761,791, 2,941,898, 3,508,947 and3,526,528 and Harasaki: “Coating technology”, p. 253, Asakura Publishing(1973). After coating, the solvent is removed if desired, and the liquidcrystal material can be obtained as an even layer.

Processes for preparing the polarization selective membrane andpolarization selective film of the present invention comprise the stepof orienting the liquid crystal composition developed on a transparentsupport to form the helical structure described above.

The method for orienting the liquid crystal composition is notspecifically limited, e.g. a smectic liquid crystal phase having ahelical structure may be obtained as the liquid crystal composition isdeveloped if it is developed at a temperature that allows the liquidcrystal composition to show the smectic liquid crystal phase having ahelical structure. The liquid crystal composition may also be orientedby once heating the liquid crystal composition developed to allow it toshow a phase appearing at a temperature higher than the temperature atwhich a smectic liquid crystal phase having a helical structure appears,e.g. to show a smectic A, chiral nematic or isotropic phase and thencooling it to a temperature at which a smectic liquid crystal phasehaving a helical structure appears. Heating can be carried out bypassing the liquid crystal composition sandwiched by two plasticsubstrates between two heated rolls or through a heat-treating furnacesimultaneously with or separately from lamination of the plasticsubstrates.

A rubbing treatment may be applied on the entire surface or a part ofthe substrate.

During the developing step or any of the subsequent steps such as theorientation step, the liquid crystal material developed between theinterfaces may be optionally subjected to a magnetic or electric field,shear stress, flow, drawing, temperature gradient, etc. Such operationshelp to shorten the step of orienting the direction of the helical axisnearly perpendicular to the plane of the polarization selective membraneand polarization selective film.

When it is necessary to inhibit any variation in the performance of thepolarization selective membrane and polarization selective film of thepresent invention with time or by heat or the like, the orientation ofthe molecules of the liquid crystalline compound is preferably fixed.

The orientation can be fixed by the step of e.g. (A) cooling theoriented liquid crystal molecules into a glass state, or (B)polymerizing the oriented liquid crystal molecules with the orientationbeing maintained.

Step (A) above can be performed by using liquid crystal moleculesshowing a smectic liquid crystal phase having a desired helicalstructure at or above the glass transition temperature and capable ofturning into a glass state by cooling, such as those based on thepolymer liquid crystal materials described above.

In step (A) above, the orientation of the liquid crystal molecules canbe fixed in a glass state without being crystallized by heating at orabove the glass transition temperature of the liquid crystal moleculesand then cooling the oriented liquid crystal molecules to a temperatureat which they turned into a glass state. The cooling means is notspecifically limited, e.g. desired enough cooling for fixing can beaccomplished only by transferring the liquid crystal molecules from theheating atmosphere used for the developing or orienting step into anatmosphere at or below the glass transition temperature such as roomtemperature. Forced cooling such as air-cooling or water-cooling may beperformed to increase the production efficiency or the like.

Step (B) above can be performed by using liquid crystal molecules havinga substituent capable of reacting by UV rays, visible light, electronrays, heat or the like. These substituents include vinyl, acryl,methacryl, vinyl ether, cinnamoyl, allyl, acetylenyl, crotonyl,aziridinyl, epoxy, isocyanate, thioisocyanate, amino, hydroxyl,mercapto, carboxylate, acyl, halocarbonyl, aldehyde, sulfonate, silanoland the like groups, preferably those groups having a multiple bond andepoxy and aziridinyl groups, more preferably acryl, methacryl, vinyl,vinyl ether, epoxy and cinnamoyl groups. In order to reduce heat- orotherwise induced variation in various performances afterpolymerization, compounds having 2 or more ethylenically unsaturatedpolymerizable groups are preferably used. Examples of compounds having 2or more ethylenically unsaturated polymerizable groups include esters ofa polyhydric alcohol and (meth)acrylic acid (e.g., ethylene glycoldi(meth)acrylate, 1,4-dicyclohexane diacrylate, pentaerythritoltetra(meth)acrylate, pentaerythritol tri(meth)acrylate,trimethylolpropane tri(meth)acrylate, trimethylolethanetri(meth)acrylate, dipentaerythritol tetra(meth)acrylate,dipentaerythritol penta(meth)acrylate, pentaerythritolhexa(meth)acrylate, 1,3,5-cyclohexanetriol triacrylate, polyurethanepolyacrylate, polyester polyacrylate), vinylbenzene and its derivatives(e.g., 1,4-divinylbenzene, 4-vinyl benzoate-2-acryloyl ethyl ester,1,4-divinylcyclohexanone), vinyl sulfones (e.g., divinyl sulfone),acrylamides (e.g., methylene bisacrylamide) and methacrylamides. Thesesubstituents may be contained in any one or more of the liquidcrystalline materials and/or non-liquid crystalline materials and/oradditives, and the substituents contained in two or more materials maybe identical and/or different. Moreover, two or more identical and/ordifferent substituents may be contained in one material.

In step (B) above, the oriented liquid crystal molecules are polymerizedwith the orientation being maintained. Suitable polymerization meansinclude, but are not specifically limited to, thermal polymerization,photopolymerization, polymerization induced by radiations such asγ-rays, electron radiation-induced polymerization, polycondensation,polyaddition and other reactions. Especially, photopolymerization usingvisible light or UV rays or electron radiation-induced polymerization ispreferably used because of the ease of controlling the reaction and theadvantage in manufacturing.

The thickness of the polarization selective membrane (or thepolarization selective layer in the polarization selective film) of thepresent invention is not specifically limited, but preferably rangesfrom 0.1 to 50 μm, more preferably 0.2 to 25 μm, still more preferably0.3 to 15 μm from the practical viewpoint.

The polarization selective membrane and polarization selective film ofthe present invention preferably have a maximum total transmittance of75% or more, more preferably 85% or more on the polarization planeperpendicular to the membrane surface. If this value is less than 75%, amarked brightness improving effect cannot be obtained because of the lowtransmittance of the membrane. The maximum total transmittance forcircularly polarized light perpendicularly incident on the membranesurface on the blocking side is preferably less than 60%, morepreferably less than 40%. If this value is 60% or more, a markedbrightness improving effect cannot be obtained, either, because thelight that should have been returned backward has been transmitted.

The polarization selective membrane of the present invention ispreferably formed as a flat layer in order to homogenize separationperformance or the like, and even when it is formed as a stack of two ormore layers, each layer is also preferably flat. As described above, thepolarization selective membrane may also be formed as a stack of two ormore layers. Stacking is advantageous for providing separation functionover a broader wavelength band or dealing with a wavelength shift ofobliquely incident light, and in this case it is preferable to stack acombination of layers reflecting non-specific circularly polarized lightat different center wavelengths. For example, a polarization selectivemembrane capable of covering a wide wavelength band can be efficientlyformed by superposing 2-6 liquid crystal layers reflecting circularlypolarized light in the same polarization direction and having centerwavelengths of selective reflection differing by 50 nm or more from eachother in the range of 300-900 nm. When liquid crystal layers aresuperposed, it is especially advantageous to use liquid crystal polymersfrom the viewpoint of production efficiency or formation of a thin film.

Methods for polarizing, polarizing plate and liquid crystal displaydevices using the polarization selective membrane and polarizationselective film of the present invention are explained below withreference to schematic figures. In the figures described below, variouscomponents of the polarization selective film and retarders may beintegrally laminated or separated. They are arranged in such a mannerthat the polarization selective film maybe interposed between the lightemerging side of the sheet-like light emitter and the retarder.

FIG. 1 is a schematic diagram showing the structure of the most basicliquid crystal display device using a polarization selective filmcontaining a polarization selective membrane of the present invention.

Liquid crystal display device 10 comprises a sheet-like emitting lightsource including a backlight source 11, a reflector 12 successivelyallowing backlights to emerge from the bottom to the top and a lightguide 13. Above the light source are arranged a polarization selectivefilm 18, a retarder 17 and a liquid crystal cell 16 sandwiched betweentwo light-absorbing polarizing plates 14 and 15 in this order. Retarder17 and light-absorbing polarizing plates 14 and 15 are arranged tomaximize the transmittance of the linearly polarized light emerging fromretarder 17.

The light from backlight source 11 is reflected by reflector 12 andguided by light guide 13 to enter polarization selective film 18 above.Specific circularly polarized light of the incident light passes throughpolarization selective film 18 and retarder 17 to the outside. On theother hand, the other circularly polarized light is reflected bypolarization selective film 18 and the reflected light is depolarized bythe light guide or the like and reflected by the reflector and returnedto polarization selective film 18 and reused.

The light reflected by polarization selective film 18 is changed in thepolarization state, whereby the reflected light partially or totallyturns into specific circularly polarized light capable of passingthrough polarization selective film 18. Thus, the reflected light isconfined between polarization selective film 18 and reflector 12 andrepeatedly reflected until it becomes specific circularly polarizedlight capable of passing through polarization selective film 18. On theother hand, the circularly polarized light emerging from polarizationselective film 18 enters retarder 17 where it is phase-shifted and thelight phase-shifted by a quarter wavelength is converted into linearlypolarized light while the light having the other wavelength is convertedinto elliptically polarized light. The elliptically polarized lightbecomes flatter as its wavelength approaches the wavelength of the lightconverted into linearly polarized light. As a result, the light rich inlinearly polarized components capable of passing through light-absorbingpolarizing plate 14 emerges from retarder 17, and the light emergingfrom retarder 17 enters liquid crystal cell 16 sandwiched betweenlight-absorbing polarizing plates 14 and 15 so that it is used todisplay images.

Thus, the utilization efficiency of the light used to display images inliquid crystal display device 10 is remarkably improved as compared withliquid crystal display devices of conventional structures (e.g. theliquid crystal display device shown in FIG. 4).

In liquid crystal display device 10, the light utilization efficiency isimproved by reusing the light reflected by polarization selective film18 as emerging light having polarization converted to prevent reflectionloss and controlling the phase of the emerging light via retarder 17 andconverting it into a light state rich in linearly polarized componentscapable of passing through light-absorbing polarizing plates 14 and 15to prevent absorption loss caused by light-absorbing polarizing plates14 and 15.

Moreover, liquid crystal display device 10 uses a polarizing systemhaving a structure comprising a sheet-like light emitter formed of abacklight source 11, a reflector 12 and a light guide 13; a polarizationselective film 18; and retarder 17. The polarizing system 19 formspolarized light likely to pass through polarizing plates with high lightutilization efficiency, as described above. It can be advantageouslyapplied as a backlight system or the like in not only liquid crystaldisplay devices but also various other equipments because it can also beformed in a large area.

FIG. 2 is a schematic diagram of a liquid crystal display device using apolarization selective film of the present invention as a protectivefilm for polarizing plates.

Liquid crystal display device 10′ shown in FIG. 2 has a structure inwhich polarization selective film 18′ and retardation film 17′ arelaminated to light-absorbing polarizing plate 14. Polarization selectivefilm 18′ and retardation film 17′ serve like polarization selective film18 and retardation film 17 in FIG. 1, respectively, and also serve asprotective films for light-absorbing polarizing plate 14. In contrast tothe liquid crystal display device shown in FIG. 1 in which the lightutilization efficiency decreases by about 10% because of the reflectionon the surface opposite to the polarization selective layer ofpolarization selective film 18 and on the surfaces of retardation film17 and light-absorbing polarizing plates 14 and 15, the lightutilization efficiency in liquid crystal display device 10′ shown inFIG. 2 increases by about 10% as compared with the liquid crystaldisplay device shown in FIG. 1 because polarization selective film 18′,retardation film 17′ and light-absorbing polarizing plate 14 arelaminated to eliminate the reflection surfaces described above.

Liquid crystal device 10′ also comprises a scattering sheet 21 and alight-collecting film 22. Scattering sheet 21 is a translucent sheetthat mainly contributes to diffusing the incident light from thesheet-like light emitter to evenly illuminate the entire surface.Light-collecting film 22 contributes to collecting the incident light tofurther improve the light utilization efficiency.

FIG. 3 shows an example of the structure of a liquid crystal displaydevice in which the brightness improving function of the polarizationselective film or polarizing plate of the present invention has beenfurther improved.

Liquid crystal display device 10″ shown in FIG. 3 further comprises ananti-reflection layer 23 laminated to the surface of the polarizationselective layer of polarization selective film 18′ directly or viaanother layer in addition to liquid crystal display device 10′ shown inFIG. 2. Reflection on the surface of the polarization selective layercan be further reduced and the amount of the light entering thepolarization selective layer can be increased as compared with liquidcrystal display device 10′ shown in FIG. 2 by laminating anti-reflectionlayer 23. Anti-reflection layer 23 may be a stack of a low-refractiveindex layer and a high-refractive index layer as described in Journal ofJapan Photographic Association, Vol. 29, P. 137 (1966), or may be asingle low-refractive index layer.

Liquid crystal display devices 10′ and 10″ shown in FIG. 2 and FIG. 3use stacks 20 and 20′ of polarization selective film 18′, retarder 17and light-absorbing polarizing plate 14 laminated in this order aspolarizing plates. The light entering light-absorbing polarizing plate14 has been converted into linearly polarized components capable ofpassing through light-absorbing polarizing plate 14 by passing throughpolarization selective film 18′ and retarder 17′, whereby the loss ofthe light entering polarization selective film 18′ and exitinglight-absorbing polarizing plate 14 is remarkably reduced as comparedwith the case where it is passed through only light-absorbing polarizingplate 14. Thus, polarizing plates 20 and 20′ can convert the incidentlight into specific polarized light with high utilization efficiency.

When the polarization selective film of the present invention is used ina liquid crystal display device, the light utilization efficiency isincreased with the result that the brightness of the display isincreased. In order to increase the brightness, the transmittance Tmaxon the polarization plane for which the total transmittance is maximumis preferably 75% or more and the transmittance Tmin on the polarizationplane for which it is minimum is preferably less than 60%, morepreferably Tmax is 80% or more and Tmin is 50% or less, especially Tmaxis 85% or more and Tmin is 40% or less.

The polarization selective film of the present invention can also beused in combination with a visual angle compensating film as describedin JP-A 1990-160204 or Japanese Patent No. 2587398.

FIGS. 1 to 3 use a sheet-like light emitter as a light source consistingof a light guide having a backlight source on the side face and areflector at the bottom, but light sources used in the present inventionare not limited to this arrangement so far as they emit light in theform of a sheet and have a reflective layer also serving as apolarization converting layer. For example, a direct backlight withoutusing a light guide can also be used.

In the present invention, the wavelength band of the light reflected asnon-specific circularly polarized light by the polarization selectivefilm is preferably substantially consistent with the wavelength band ofthe output light originating from the sheet-like light emitter used incombination as a light source. When the output light contains dominantwavelengths such as bright line spectra, it is more advantageous for theefficiency of selecting polarized light to match one or more suchdominant wavelengths to the wavelength of the reflected lightoriginating from the smectic liquid crystal phase or the like of thepolarization selective film. When the polarization selective membrane isformed in a stack structure, not only abroad wavelength band of lightcan be dealt with but also the necessary number of layers to besuperposed can be reduced to provide a benefit for decreasing thethickness of the polarization selective layer. The wavelength of thereflected light is preferably consistent with one or more dominantwavelengths of the sheet-like light emitter within the range of 20 nm.

In the present invention, the retarder provided above the polarizationselective film has the purpose of changing the phase of the circularlypolarized light emerging from the polarization selective film to convertit into a state rich in linearly polarized components more likely topass through polarizing plates. Thus, retarders can be preferably usedcapable of converting the circularly polarized light emerging from thepolarization selective film into a state rich in linearly polarizedlight to correspond to a phase change of a quarter wavelength and alsoconverting the light of the other wavelength into flat ellipticallypolarized light having a major direction as possible as parallel to thelinearly polarized light and as possible as close to the linearlypolarized light.

By using a retarder as described above, the direction of the linearlypolarized light of the output light or the direction of the major axisof the elliptically polarized light can be arranged as possible asparallel to the transmission axis of the polarizing plate to give lightrich in linearly polarized components capable of passing through thepolarizing plate.

Preferably, the retarder used in the present invention can be formed byan appropriate material to provide a transparent retarder giving an evenphase change. The phase change in the retarder can be appropriatelydetermined depending on the wavelength band of the circularly polarizedlight emerging from the polarization selective film and other factors.In the visible region, retarders giving a small phase change,specifically a phase change of 100-200 nm can often be preferably usedfrom the viewpoint of wavelength characteristics and practicability,also considering that most retarders show positive wavelength dispersionof birefringence because of the characteristics of the material.

The retarder can be formed in one or more layers. In the case ofretarders consisting of a single layer, those showing smaller wavelengthdispersion of birefringence are preferred for homogenization of thepolarization state of each wavelength. Stacked retarders are effectivefor improving wavelength characteristics in the wavelength band, and thecombination can be appropriately determined depending on the wavelengthband or other factors.

When left-handed circularly polarized light enters a retarder giving aphase change of 100 to 200 nm as described above, the transmission ofthe polarizing plate can be improved by orienting the fast axis of theretarder at an angle of 0 to 90°, preferably 35 to 55°, especially 45°with respect to the polarization axis of the polarizing plate (0°). Whenright-handed circularly polarized light enters the retarder, thetransmission of the polarizing plate can be improved by orienting theslow axis of the retarder at the angle described above. When theretarder consists of 2 or more layers, especially when the outer surfacelayer is occupied by a layer giving a phase change of 100 to 200 nm,these angles in such a layer are preferably decided as described above.

When a retarder consisting of 2 or more layers is used in the visibleregion, one or more odd number of layers giving a phase change of 100 to200 nm are preferably contained for obtaining light rich in linearlypolarized components. Layers other than those giving a phase change of100 to 200 nm are preferably formed by, but not limited to, layersgiving a phase change of normally 200 to 400 nm in terms of theimprovement in wavelength characteristics or the like.

EXAMPLES

The present invention will further be detailed referring to specificExamples. It is to be noted that any materials, reagents, ratios of usethereof and operations shown in the Examples below can properly bemodified without departing from the spirit of the present invention.Thus the present invention is by no means limited to the Examplesdescribed below.

Example 1

In this example, the temperature dependences of the wavelength ofselective reflection of cholesteric liquid crystals and chiral smecticliquid crystals are compared to demonstrate advantages of the presentinvention using a chiral smectic phase.

Liquid crystal molecules consisting of a mixture of specific compounds(108), (109) and (110) in equal mass were mixed with 30% by mass of achiral agent (CE7 from BDH), and the mixture was heated to 130° C. andthen cooled at a rate of −5° C./min with the result that a cholestericphase appeared at 119.4° C. (T_(N)) and a chiral smectic C (Sc*) phaseappeared at 60.9° C. (Tc). Thus, it was found that the mixture of theseliquid crystals and chiral agent shows a cholesteric phase and a chiralsmectic phase with the same composition, and this mixture was used toexamine the temperature dependence of the wavelength of selectivereflection (in terms of the helical pitch length) of each phase.

Experiments were performed by injecting the mixture into a horizontallyoriented cell having a cell gap of 50 μm (KSRO-50/A511N7NSS(ZZ) fromEHC) and measuring the helical pitch length at the temperatures 5° C.and 15° C. lower than the transition temperatures to the cholestericphase and chiral smectic phase (T_(N), T_(C)) during cooling todetermine the rate of change. As a result, the pitch length changed by27% in cholesteric phase but only 19% in the chiral smectic phase,showing that the temperature dependence of the pitch length is smallerin the chiral smectic phase. This result shows that the selectivereflection wavelength of the chiral smectic phase is lesstemperature-dependent when a liquid crystal layer selectively reflectinga specific wavelength band is formed, which is favorable for controllingthe temperature during the formation of the liquid crystal layer andgreatly advantageous for manufacturing processes.

When a chiral smectic phase was formed at 55° C., the angle formed bythe orientation of the long axis of the liquid crystals and the normaldirection of the cell was 22°.

Example 2

In this example, the angle dependences of the wavelength of selectivereflection of cholesteric liquid crystals and chiral smectic liquidcrystals are compared to demonstrate advantages of the present inventionusing a chiral smectic phase.

(Determination of Transmittance)

A spectrophotometer UV-3100PC from Shimadzu was used to determineselective reflection of circularly polarized light. A test system wasformed by arranging a light source, an absorptive-type linear polarizingplate (HLC-5618S from Sanritz), a λ/4 plate (PURE-ACE WR, W-159 fromTeijin), a sample and a receiver in this order. A baseline wasestablished by replacing the sample with a glass plate. For a sampleselectively reflecting right-handed circularly polarized light, thetransmittance on the transmission side was determined by measuring thetransmittance when left-handed circularly polarized light was enteredand the transmittance on the blocking side was determined by measuringthe transmittance when right-handed circularly polarized light wasentered. For a sample selectively reflecting left-handed circularlypolarized light, the transmittance on the transmission side wasdetermined by measuring the transmittance when right-handed circularlypolarized light was entered and the transmittance on the blocking sidewas determined by measuring the transmittance when left-handedcircularly polarized light was entered. The incident right-handedcircularly polarized light and left-handed circularly polarized lightwere created by rotating the slow axis of a λ/4 plate next to theabsorptive linear polarizing plate by 90°.

(Preparation of Circular Polarizing Elements)

A solution of 10% by mass of liquid crystals forming a chiral smectic Cphase (liquid crystal FLC-6304 from Rolic) in chloroform was applied ona polyethylene terephthalate film and dried. The thickness of theFLC-6304 layer after drying was 2 μm. Then, the temperature was raisedto 120° C. and then lowered to room temperature at a rate of −5° C./minto prepare a circular polarizing element formed of a chiral smectic Cphase (H-1). When the liquid crystals formed a chiral smectic C phase,the angle formed by the orientation of the long axis of the liquidcrystals and the normal direction of the cell was 22°.

As a comparative sample, a solution of 10% by mass of liquid crystalsforming a cholesteric phase (a mixture of 85% by mass of DON-103 fromDainippon Ink and 15% by mass of a commercially available chiral agent)in chloroform was applied on a polyethylene terephthalate film anddried. The thickness of the liquid crystal layer after drying was 2 μm.Then, the temperature was raised to 120° C. and then lowered to roomtemperature at a rate of −5° C./min to prepare a circular polarizingelement formed of a cholesteric phase (R-1).

(Preparation of Linear Polarizing Elements)

Thus prepared circular polarizing element (H-1) and a λ/4 plate(PURE-ACE WR, W-159 from Teijin) were laminated to each other to preparea linear polarizing element (HA-1). As a comparative sample, thecircular polarizing element (R-1) and a λ/4 plate (Teijin) werelaminated to each other to prepare a linear polarizing element (RA-1).

(Evaluation of the Incident Angle Dependence of the Transmittance)

The linear polarizing element (HA-1) of the present invention showed theminimum transmittance at a wavelength of 450 nm when light was enteredat right angles to the polarizing element surface, while it showed theminimum transmittance at a wavelength of 380 nm when light was enteredfrom the direction of angle of 45° with respect to the polarizingelement surface. However, experiments using the linear polarizingelement (RA-1) prepared as a comparative sample showed the minimumtransmittance at a wavelength of 450 nm when light was entered at rightangles to the polarizing element surface and the minimum transmittanceat a wavelength of 350 nm when light was entered from the direction ofangle of 45° with respect to the polarizing element surface. This showsthat the incident angle dependence of the transmittance is smaller inthe system using a chiral smectic phase of the present invention thanthe system using a cholesteric phase. When a polarizing element ispractically mounted on a liquid crystal display, the entire visiblelight must be covered, which means that high dependence on the incidentangle invites disadvantages such as increased thickness and increasedcosts because a wider range must be ensured for the long wave side. Itis obvious from this point that the system using a chiral smectic phaseless dependent on the incident angle is excellent.

Example 3

In this example, an optical film of the present invention is mounted ona liquid crystal display (LCD) to demonstrate that it is useful as abrightness-improving film.

(Preparation of a Circular Polarizing Element)

Similarly to Example 2, a solution of 10% by mass of FLC-6304 inchloroform was applied on a polyethylene terephthalate film and dried.The thickness of the liquid crystal layer after drying was 10 μm. Then,the temperature was raised to 120° C. and then lowered to roomtemperature at a rate of −5° C./min to prepare a circular polarizingelement. The angle formed by the orientation of the long axis of theliquid crystals of the chiral smectic phase and the normal direction ofthe cell was 22°.

(Preparation of Linear Polarizing Elements)

Thus prepared circular polarizing element and a λ/4 plate (PURE-ACE WR,W-159 from Teijin) were laminated to each other to prepare a linearpolarizing element.

This linear polarizing element was used as a protective layer on oneside, and followed by an absorptive polarizing layer of iodine/PVA andthen a protective layer formed of a triacetylcellulose film to prepare alinear polarizing element integrated with an absorptive polarizinglayer.

(Determination of the Transmittance)

When right-handed circularly polarized light having a wavelength of 450nm was entered into this linear polarizing element integrated with anabsorptive polarizing layer, a transmittance of 92% was obtained. Whenleft-handed circularly polarized light having a wavelength of 450 nm wasentered, a transmittance of 35% was obtained.

(LCD Packaging)

Thus prepared linear polarizing element integrated with an absorptivepolarizing layer was used as a polarizing plate on the light source sideof an LC cell and mounted on a liquid crystal display device made bySHARP. The intensity of the wavelength of 450 nm in the front directionwhen using this linear polarizing plate integrated with an absorptivepolarizing layer was measured with a spectroradiometer SR-2 made byTOPCON to give a brightness 1.10 times stronger than obtained withnormal absorptive polarizing plates, confirming that the optical film ofthe present invention serves as a brightness-improving membrane for LCD.

Example 4

This example demonstrates that liquid crystal compositions containing acompound of formula (I) above show a chiral smectic phase and that saidphase shows selective reflection. A mixture containing specificcompounds (108), (109) and (110) in equal mass was mixed with specificcompounds 1 and 3 and comparative compounds 1 (having a structure shownbelow) and 2 (CE7 from BDH) each at a content of 5% by mass to prepareliquid crystal compositions A, B, C and D.

When these liquid crystal compositions were heated to a temperature atwhich they became isotropic liquids and then cooled at a rate of −5°C./min, selective reflection was observed in a chiral smectic C phase inliquid crystal compositions A and B. Phase-separated texture wasobserved in liquid crystal composition C, while no selective reflectionwas shown by liquid crystal composition D.

Then, liquid crystal composition E was prepared in the same manner asfor liquid crystal composition D except that compound 2 was contained at30% by mass. When this liquid crystal composition E was heated to atemperature at which it became an isotropic liquid and then cooled at arate of −5° C./min, selective reflection was observed.

Thus, specific compounds (1) and (3) were shown to be well compatiblewith the smectic phase and induce a helical structure having a shortpitch enough to selectively reflect visible rays at such a low contentas 5% by mass.

Liquid crystal compositions A and B were injected into a horizontallyoriented cell having an ITO electrode with a cell gap of 7 μm and heatedto a temperature at which the compositions showed a chiral smectic Cphase. When dc voltage was applied in the normal direction of the cellplane and the electric field was inverted, the angle at which theextinction position was rotated was measured to show that the angleformed by the orientation of the long axis of the liquid crystals andthe helical direction was 22 to 30° in both compositions.

Example 5

In this example, an optical film using a liquid crystal composition ofthe present invention was mounted on a liquid crystal display (LCD) todemonstrate that it is useful as a brightness-improving film.

(Preparation of Circular Polarizing Elements)

A solution of 10% by mass of liquid crystal composition A above inchloroform was applied on a polyethylene terephthalate film and dried.The thickness of the liquid crystal layer after drying was 10 μm. Then,the temperature was raised to 120° C. and then lowered to roomtemperature at a rate of −5° C./min to prepare circular polarizingelement A.

Circular polarizing element B was prepared exactly in the same mannerexcept that liquid crystal composition A was replaced by liquid crystalcomposition B.

(Preparation of Linear Polarizing Elements)

Thus prepared circular polarizing elements A and B were each laminatedto a λ/4 plate (PURE-ACE WR, W-159 from Teijin) to prepare linearpolarizing elements A and B.

These linear polarizing elements A and B were each used as a protectivelayer on one side, and followed by an absorptive polarizing layer ofiodine/PVA and then a protective layer formed of a triacetylcellulosefilm to prepare linear polarizing elements A and B integrated with anabsorptive polarizing layer.

(Determination of the Transmittance)

When right-handed circularly polarized light having a wavelength of 450nm was entered into these linear polarizing elements A and B integratedwith an absorptive polarizing layer, transmittances of 91% and 90% wereobtained respectively. When left-handed circularly polarized lighthaving a wavelength of 450 nm was entered, transmittances of 34% and 33%were obtained respectively.

(LCD Packaging)

Thus prepared linear polarizing elements A and B integrated with anabsorptive polarizing layer were each mounted on a liquid crystaldisplay device made by SHARP as a polarizing plate on the light sourceside of an LC cell. The intensity of the wavelength of 450 nm in thefront direction when using these linear polarizing plates integratedwith an absorptive polarizing layer was measured with aspectroradiometer SR-2 made by TOPCON to give a brightness 1.09 timesstronger in linear polarizing element A and 1.10 times stronger inlinear polarizing element B than obtained with normal absorptivepolarizing plates, confirming that the optical films of the presentinvention serve as brightness-improving membranes for LCD.

Having described our invention as related to the present embodiments, itis our intention that the invention not be limited by any of the detailsof the description, unless otherwise specified, but rather be construedbroadly within its spirit and scope as set out in the accompanyingclaims.

1. A polarization selective membrane comprising a layer formed from acomposition comprising at least one liquid crystal molecule having ahelical structure and selectively transmitting specific polarized lightand selectively reflecting the other polarized light, wherein thehelical axis of the helical structure is practically oriented in thenormal direction of the membrane plane and the angle formed by theorientation of the helical axis of the helical structure and theorientation of the long axis of the liquid crystal molecule is from 5°to 85°.
 2. The polarization selective membrane of claim 1, whereinmaximum total transmittance is 75% or more on the polarization planeperpendicular to the membrane surface.
 3. The polarization selectivemembrane of claim 1, having a maximum total transmittance of 75% or moreand a minimum total transmittance of less than 60% at a polarizationplane perpendicular to the membrane plane.
 4. The polarization selectivemembrane of claim 1, wherein the liquid crystal molecule exhibits achiral smectic phase.
 5. The polarization selective membrane of claim 1,wherein the composition comprising at least one liquid crystal moleculefurther comprises an optically active compound of formula (I) below:

wherein X represents a group having an optically active site, Mrepresents a group having at least one aromatic carbocycle or aromaticheterocycle, Y¹ and Y² independently represent —O—, —S—, —C(═O)O—,—OC(═O)—, —OC(═O)O—, —C(═O)N(R¹)—, —N(R¹)C(═O)—, —(CR²R³)_(m)O—,—SO₂N(R¹)—, —N(R¹)SO₂— or —S(═O)_(p)—; R¹, R² and R³ independentlyrepresent a hydrogen atom or an optionally substituted alkyl group; mrepresents an integer of any of 1 to 12, p represents an integer of anyof 0 to 2, m¹ and m² independently represents 0 or 1, provided that whenm¹ and m² are each 0, M and X and M and R are directly bonded; Rrepresents an optionally substituted alkyl group and n represents aninteger of any of 2 to
 6. 6. The polarization selective membrane ofclaim 5, wherein X in Formula (I) is a cyclic group having an opticallyactive site.
 7. The polarization selective membrane of claim 5, whereinX in Formula (I) is a cyclic group selected from Group I:


8. The polarization selective membrane of claim 5, wherein M in Formula(I) is a group represented by Formula (II) below:

wherein T¹ and T² independently represent an aromatic carbocycle,aromatic heterocycle or aliphatic carbocycle, and at least onerepresents an aromatic carbocycle or an aromatic heterocycle; Y³independently represents —O—, —C(═O)O—, —OC(═O)—, —CH₂O—, —OCH₂—, —CH═N—or —N═CH—; m³ represents 0 or 1, provided that when m³ is 0, T¹ and T²are directly bonded; and r represents an integer of any of 0 to
 3. 9.The polarization selective membrane of claim 5, wherein the liquidcrystal molecule exhibits a chiral smectic phase.
 10. The polarizationselective membrane of claim 5, wherein the amount of an optically activecompound of formula (I) is from 1 to 30% by weight based on the totalamount of composition.
 11. A polarization selective film comprising: atransparent support and a layer selectively transmitting specificpolarized light and selectively reflecting the other polarized light,formed from a composition comprising at least one liquid crystalmolecule having a helical structure; wherein the helical axis of thehelical structure is practically oriented in the normal direction of thelayer plane and the angle formed by the orientation of the helical axisof the helical structure and the orientation of the long axis of theliquid crystal molecule is from 5° to 85°.
 12. A polarizing systemcomprising: a polarization selective film comprising a transparentsupport and a layer selectively transmitting specific polarized lightand selectively reflecting the other polarized light, formed from acomposition comprising at least one liquid crystal molecule having ahelical structure of which helical axis is practically oriented in thenormal direction of the layer plane and the angle formed by theorientation of the helical axis of the helical structure and theorientation of the long axis of the liquid crystal molecule is from 5°to 85°; a retarder having a phase change of 100-200 nm, and alight-absorbing polarizing plate laminated in this order, wherein thedirection having a polarization plane on which the total transmittanceof the polarization selective film is maximum and the direction of thetransmission axis of the light-absorbing polarizing plate issubstantially parallel.
 13. A liquid crystal display device comprising:a backlight, a polarization selective film comprising a transparentsupport and a layer selectively transmitting specific polarized lightand selectively reflecting the other polarized light, formed from acomposition comprising at least one liquid crystal molecule having ahelical structure of which helical axis is practically oriented in thenormal direction of the layer plane and the angle formed by theorientation of the helical axis of the helical structure and theorientation of the long axis of the liquid crystal molecule is from 5°to 85°, a retarder; and a liquid crystal cell sandwiched between a pairof light-absorbing polarizing plates arranged in this order; wherein thebacklight comprises a light guide having a light source on the side faceand a reflective layer at the bottom, and the retarder has a phasechange of 100 to 200 nm.
 14. A liquid crystal composition comprising atleast one liquid crystal showing a smectic phase and an optically activecompound of formula (I) below, wherein the composition is capable offorming a helical structure in which the angle formed by the orientationof the helical axis and the orientation of the long axis of the liquidcrystal molecules is from 5° to 85°:

wherein X represents a group having an optically active site, Mrepresents a group having at least one aromatic carbocycle or aromaticheterocycle, Y¹ and Y² independently represent —O—, —S—, —C(═O)O—,—OC(═O)—, —OC(═O)O—, —C(═O)N(R¹)—, —N(R¹)C(═O)—, —(CR²R³)_(m)O—,—SO₂N(R¹)—, —N(R¹)SO₂— or —S(═O)_(p)—; R¹, R² and R³ independentlyrepresent a hydrogen atom or an optionally substituted alkyl group; mrepresents an integer of any of 1 to 12, p represents an integer of anyof 0 to 2, m¹ and m² independently represents 0 or 1, provided that whenm¹ and m² are each 0, M is directly bonded to each of X and R; Rrepresents an optionally substituted alkyl group and n represents aninteger of any of 2 to
 6. 15. The composition of claim 14, wherein X inFormula (I) is a cyclic group having an optically active site.
 16. Thecomposition of claim 14, wherein X in Formula (I) is a cyclic groupselected from Group I:


17. The composition of claim 14, wherein M in Formula (I) is a grouprepresented by Formula (II) below:

wherein T¹ and T² independently represent an aromatic carbocycle,aromatic heterocycle or aliphatic carbocycle, and at least onerepresents an aromatic carbocycle or an aromatic heterocycle; Y³independently represents —O—, —C(═O) O—, —OC(═O)—, —CH₂O—, —OCH₂—,—CH═N— or —N═CH—; m³ represents 0 or 1, provided that when m³ is 0, T¹and T² are directly bonded; and r represents an integer of any of 0 to3.
 18. The composition of claim 17, wherein r in formula (II) is 0 or 1.19. The composition of claim 14, wherein the composition exhibits achiral smectic phase.
 20. The composition of claim 14, wherein theamount of an optically active compound of formula (I) is from 1 to 30%by weight based on the total amount of composition.
 21. The compositionof claim 14, wherein the at least one liquid crystal shows a chiralsmectic phase.
 22. The composition of claim 14, wherein the at least oneliquid crystal shows a chiral smectic C phase.
 23. The composition ofclaim 14, wherein the at least one liquid crystal is selected from thegroup consisting of compounds (101) to (116) shown below, andcombinations thereof: